Question: Which of these numbers is prime? ${5,\ 78,\ 87,\ 95,\ 99}$
A prime number has exactly two factors: 1 and itself. The factors of 5 are 1 and 5. The factors of 78 are 1, 2, 3, 6, 13, 26, 39, and 78. The factors of 87 are 1, 3, 29, and 87. The factors of 95 are 1, 5, 19, and 95. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 5 is a prime number.